The disappearance of a glacier high in the Andes could provide the clearest evidence yet of global climate change, a senior academic has warned.

The Peruvian Qori Kalis glacier could vanish in just five years, climatologist Lonnie Thompson predicts.

The fragile sliver of ice is one of many dripping from the Quelccaya Ice Cap, the largest tropical body of ice.

He told a conference in San Francisco that new observations showed the melting of Qori Kalis was accelerating.

"I would not be surprised to see half of it disappear in this coming year," Professor Thompson, from Ohio State University, told the BBC.

Rapid change

Professor Thompson has been studying Qori Kalis since 1978.

"In the first 10 years we observed the glacier it was retreating 6m (19.7ft) every year," he said. "In the last few years, it has started retreating 60m (197ft) every year - a 10-fold increase."

As the glacier has shrunk, it has exposed ancient plants that have not seen the light of day for 5,000 years, showing the current retreat exceeds anything seen before in that timeframe.

Professor Thompson said the dramatic change in the Andes had the fingerprint of man-made climate change.

The retreat of glaciers is going to affect people dependent on ice melt

But in 2007, additional factors will compound the situation, he added.

"On top of that you will have natural phenomena like El Nino, which release heat into the lower atmosphere," he predicted.

El Nino events are marked by the arrival of unusually warm waters off the north-western coast of South America, and are described as the largest influence on the year-to-year variability of the Earth's climate.

Once the warm air from these currents makes its way into the atmosphere, it is rapidly distributed throughout tropical regions.

"The combination of those two things will have a big impact on glaciers throughout the tropics," Professor Thompson observed.

Global problem

The same signature of rapid retreat seen in Peru is also being witnessed around the world.

Millions of people are going to have to adapt to these changes, many of which will occur in some of the poorest regions of the globe

Professor Lonnie Thompson,US climatologist

Professor Thompson has recently returned from Naimona'nyiin in Tibet, which is showing a similar pattern.

And regardless of what we do to mitigate climate change, he believes that some tropical glaciers are now doomed.

"No matter what we do, we are going to lose the glaciers on Kilimanjaro and the lower elevation glaciers in the Andes.

"Kilimanjaro could be gone by 2020," Professor Thompson suggested. "In the Andes, some of the glaciers are bigger, but I think we are talking 30 to 50 years."

As this happens it will cause severe humanitarian problems, he said.

"Millions of people are going to have to adapt to these changes, many of which will occur in some of the poorest regions of the globe."

Dry lands

Of particular concern is the boom-and-bust scenario for water supplies in areas fed by these glaciers.

As the ice melts, producing a lot of water, the local populations become reliant on it for agriculture, and hydroelectricity.

But when the glaciers are gone, so too is the water.

"These changes are going to take place and these people will be impacted," said Professor Thompson. "They have to find ways to adapt."

The research was presented at the American Association for the Advancement of Science's (AAAS) annual meeting in San Francisco.